Installation and method for manufacturing cross-linkable polyethylene compounds

ABSTRACT

An installation for manufacturing cross-linkable polyethylene compounds which comprises a melting machine (101), a melt pump (102) and a filtration unit (103) to produce cross-linkable polyethylene compounds that may be further used for manufacturing insulating parts of medium, high and extra-high voltage power cables, and a method for manufacturing such cross-linkable polyethylene compounds.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Division of US application Ser. No. 16/070,472 filed Jul. 16,2018, allowed, which is an US § 371 National Stage of PCT/IB2017/050117filed Jan. 11, 2017, published as WO 2017/122122 A1 on Jul. 20, 2017,the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The invention concerns the technical field of installations formanufacturing polymer compounds. The invention is directed in particularto an installation for manufacturing cross-linkable polyethylenecompounds and to a method for manufacturing cross-linkable polyethylenecompounds.

STATE OF THE ART

Cross-linkable polyethylene (XLPE) is used for manufacturing insulationparts of power cables, in particular medium, high and extra-high voltagepower cables. Because power cables are usually buried in the ground forseveral decades, insulation parts must show constant chemical andmechanical properties over long periods of time. One of those requiredproperties, heat aging stabilization, is linked to persistence over timeof optimum insulation and electrical properties and is of paramountimportance.

The document US 20091 10833 provide an example of installation for themanufacturing of cross-linkable polymer compounds. EP 2918388 providesanother example of installation for recycling plastic materials.

In order to produce cross-linkable polyethylene that can allow suchrequired properties to be provided, in particular to producehigh-quality cross-linkable polyethylene that respects the technical andlegal standards allowing it to be used for manufacturing insulationparts of medium, high or extra-high voltage power cables, it isnecessary to conduct a process that mixes raw material, for examplelow-density polyethylene (LDPE), additives and peroxide.

Usually, this process is implemented by making use of a so-calledConventional Medium—High—Extra High Voltage—Cross-linkable PolyethyleneCompound Production Line which is a mature technology that started inthe 60's. Such production lines always require a reasonably large plantwhere, in order to ensure the manufacturing environment required by theprocess that is fulfilled, a high-level of cleanliness must bemaintained. This high level of cleanliness implies the use of so-called“clean rooms”, in which technologies to avoid presence of contaminantsare provided, and the provision of several technologies to constantlymonitor and maintain the required high cleanliness level.

In terms of the process which is implemented, state of the artproduction lines or installations, when they are designed to producecross-linkable polyethylene that respects the technical and legalstandards allowing it to be used for manufacturing insulation parts ofmedium, high or extra-high voltage power cables, materialize theprinciple that, since the peroxide, which has to be incorporated at somestage of the process, is not able to stand the required compoundingtemperatures, it must be subsequently incorporated during a so-calledsoaking process.

This principle implies that traditional production plants, in particularthose designed to produce cross-linkable polyethylene that respects thetechnical and legal standards allowing it to be used for manufacturinginsulation parts of medium, high or extra-high voltage power cables, arealways based on a batch peroxide soaking process, with controlledheating, cooling and residence time sequences. This soaking process isalways conducted in an enclosed soaking tower which height is usuallyaround 50 m, such high height being necessary because the chain ofoperations conducted inside the tower relies on gravity.

Therefore, state of the art installations for manufacturing high-qualitycross-linkable polyethylene present a first disadvantage because,relying on a specific process, they must always have at least one veryhigh soaking tower. However, depending on the area of the world, it isnot always easy to obtain a construction permit for erecting such anelevated tower. Moreover, there are definitely some regions in the worldwhere building a tower of a relatively high height is strictlyforbidden.

In terms of costs, state-of-the-art installations for manufacturing highquality cross linkable compounds present a second disadvantage becausethey are pretty expensive, with a price ranging between 15 to 20 millionUSD which includes the land, a complete compounding unit, the soakingunit and associated buildings. These figures are also inherent to thecountry where the global unit is erected. Obviously, such a big capitalinvestment is largely impacted by the cost of erecting the soakingtowers.

In addition to the need of a soaking tower and associated cost,traditional production lines present also other disadvantages such as anon-homogeneous dispersion of the peroxide, a high risk of externalcontamination, the need to have many clean rooms and associateddedicated people, the difficulty to maintain high-levels of cleanlinessand further disadvantages in terms of energy consumption, whichobviously leads to relatively large carbon footprint.

SUMMARY OF THE INVENTION

The present invention provides a new installation for manufacturingcross-linkable polyethylene compound without any drawbacks known in theart.

In particular, the present invention intends to provide an installationfor manufacturing cross-linkable polyethylene, in particularcross-linkable polyethylene which quality fulfills the legal andtechnical requirements allowing it to be used for manufacturinginsulation parts of medium, high or extra-high voltage power cables,which installation does not need any soaking tower.

The invention also provides an installation for manufacturing highquality cross-linkable polyethylene which is less expensive to build.

These goals are achieved by an installation for manufacturing across-linkable polyethylene compound, the installation defining aproduction path starting at an entry point where raw material isacquired, the installation comprising a first melting machine (101), amelt pump (102) and a filtration unit (103), and the installationfurther comprises, in a direction starting at the entry point and goingalong said production path, either a cooler (104) arranged after thefiltration unit (103), a mixer (106) arranged after the cooler (104) andan additive dispensing unit (105) connected with the first meltingmachine (101), with the cooler (104), with the mixer (106) and/or withconnection means that connect together the cooler (104) and the mixer(106), or a second melting machine (111) arranged after the filtrationunit (103) and an additive dispensing unit (105) connected to the secondmelting machine (111), or a post extrusion additive dispensing unit(112) arranged after the filtration unit (103) and an additivedispensing unit (105) connected to the post extrusion additivedispensing unit (112), and a method for manufacturing a cross-linkablepolyethylene compound, comprising:

-   -   using a first melting machine (101), which output is connected        to a melt pump's (102) input;    -   using a filtration unit (103), which input is connected to the        melt pump's (102) output; wherein the method further comprises:    -   using either a cooler (104), which input is connected to the        filtration unit's (103) output, a mixer (106), which input is        connected to the cooler's (104) output, and an additive        dispensing unit (105), which output is connected with the        melting machine (101), with the cooler (104), with the mixer        (106) and/or with connection means that connect together the        cooler and the mixer; or    -   using a second melting machine (111), which input is either        connected to the filtration unit (103) or to drying unit (109),        which input is connected to an output of an underwater        pelletizing unit, which input is connected to an output of the        filtration unit (103).

By making use of specific components arranged in a specific order, theinstallation according to the invention, in particular when it must beused for manufacturing cross-linkable polyethylene which qualityfulfills the legal and technical requirements allowing it to be used formanufacturing insulation parts of medium, high or extra-high voltagepower cables, does not need any soaking tower. Indeed, the presence ofspecific components arranged in a specific order allows to decrease thepolymer compound temperature from a high level of temperature that hasto be set on the melting machine, so as to allow the very finefiltration level required for the production of cross-linkablepolyethylene that can be used for manufacturing insulation parts ofmedium, high and extra-high voltage power cables, to a lower level whichallows peroxide to be incorporated into the compound without initiatingthe decomposition reaction.

Further advantages are provided by specific embodiments as described indetail hereinbelow.

DESCRIPTION OF THE FIGURES

Objects and advantages of the invention will be realized by the personskilled in the art by reading the following detailed description ofseveral embodiments which should be taken in conjunction with thedrawings wherein:

FIG. 1 diagrammatically shows an installation according to a firstembodiment of the invention;

FIG. 2 diagrammatically shows an installation according to a secondembodiment of the invention;

FIG. 3 diagrammatically shows an installation according to a thirdembodiment of the invention;

FIG. 4 diagrammatically shows an installation according to a fourthembodiment of the invention;

FIG. 5 diagrammatically shows an installation according to a fifthembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As may be seen on the figures, the installation 100 for manufacturingcross-linkable polyethylene compound according to the invention definesa production path that starts at an entry point of the installation,where raw material is acquired. Depending on the end product to beoutput by the installation, the raw material may be low-densitypolyethylene (LDPE), very low-density polyethylene (VLDPE), EthylenePropylene Rubber (EPR), ethylene-vinyl acetate (EVA), ethylene propylenediene monomer (EPDM M-class) rubber or other copolymers (EVA, EEA, EMA,EBA) or any of the above-mentioned polymers and co-polymerspre-stabilized with one or more antioxidants or a composition thereof.Each of the above-mentioned polymers and co-polymers may be neat. Theymay be used alone or in combination with other polymers or co-polymers.The raw material may be produced at the same location of theinstallation, delivered to the installation through a pipe connected toa reactor area, via one or more intermediate buffers or silos, or it maybe delivered through palettes of, usually, 25 kg bags. Usually, rawmaterial is preferably delivered to the installation in the form ofpellets or even under powder form.

Along the production path, the installation 100 according to theinvention comprises several components arranged in a specific order. Allcomponents along the production path are connected together byconnection means such as pipes, tubes or any other equivalent connectionmeans well known in the art of polymer compound production plants.

According to a first embodiment of the invention, shown on FIG. 1 theinstallation comprises a melting machine 101 which acquires, melts andmixes raw material. The melting machine 101 allows preparing polymericformulations by heating, mixing and/or blending polymers and additivesin a molten state. These polymers and additives are automatically dosedwith fixed set points usually through feeders also called loss-in-weightfeeders or volumetric feeder or even side feeders. The melting machine101 is meant to output a homogeneous molten compound in whichdistribution and dispersion of additives, if present, as well as shearrate and temperature of the molten compound are optimal.

The melting machine 101 is made to heat the polymer compound at a presettemperature and is equipped with several inlets to allow mixing ofdifferent polymers or adding additives, for example antioxidant orconductive carbon black, to the polymer compound whenever a specialformulation is required. For example, water tree resistant compounds orDC (Direct Current) compounds or semiconductor compounds might requireadding several polymers and/or solid or liquid ingredients. Shouldadditives, with a concentration between zero and 50%, be added to theformulation, no nitrogen blanket is required.

The melting machine 101 may be an internal mixer, a twin-screwco-rotating extruder, a twin-screw counter-rotating extruder, acontinuous mixer, a co-kneader or any other type of melting machineknown in the art of polymer compound production.

Optionally, a debagging unit (not shown), which role is to open anddischarge bags of raw material to feed the melting machine 101, isarranged before the melting machine 101.

Once the compound has gone through the melting machine 101, it is pumpedby a melt pump 102 and pressed through a filtration unit 103. Themelting temperature set on the melting machine 101, when the desiredproduct provided by the installation is cross-linkable polyethylenewhich quality fulfills the legal and technical requirements allowing itto be used for manufacturing insulation parts of medium, high orextra-high voltage power cables, depends on the filtration level, but isgenerally around 200° C. This is well above the temperature of thedecomposition of the peroxides. As the skilled person knows, such hightemperature forbids to incorporate peroxide at this stage of the processand, therefore, peroxide has to be incorporated farther on during theprocess as will be described below.

The input of the melt pump 102 is connected directly to the output ofthe melting machine 101. The melt pump builds pressure and provides aconsistent volume of output to the filtration unit 103. The gears of themelt pump 102 are filled by the melting machine 101 from the suctionside and the melt pump 102 discharge a constant volume of melt to thefiltration unit 103. This process is continuous.

The filtration unit 103 filters the compound coming out the melt pump102. Preferably, the filtration unit's input is connected directly tothe melt pump's output.

Preferably, the filtration unit 103 ensures filtration at 35 mesh to 500mesh, which ensures filtration of sizes between 25 pm to 500 pm. Thefiltration unit 103 may provide a larger range of filtration dependingon the requested quality of the insulating product, related either tomedium voltage or extra high voltage. Thus, the filtration range canspan from about 10 microns up to about 500 microns.

Preferably, the filtration unit 103 includes a screen changertechnology, such as continuous plate, rotating screen changer, slideplate screen changer or any candle filter with woven or none wovenfilter medium able to stop particles which size ranges from 10 pm to 500pm, preferably 25 pm to 500 pm, as well as high polyethylene molecularweights known as Gels.

In contrast to prior art installations, once the compound has gonethrough the filtration unit 103, it enters a cooler 104. The role of thecooler 104 is to decrease the temperature of the compound to a levelwhich allows peroxide to be incorporated into the compound withoutinitiating the peroxide scission reaction. The cooler 104 is merely oneof several melt coolers well known in the art. The cooler's principle issuch that the melt flows in a single stream through a dense arrangementof pins or tooth meanwhile a cooling media flows inside the pins or thetooth as well as the mantel jacket. This induces a high heat exchangearea which allows effective cooling over a relatively short length.

Further, the cooler 104 is connected to a mixer 106. Preferably, thecooler's 104 output is directly connected to the mixer's 106 input.Alternatively, the cooler's output is connected to the mixer's input viaadditional connection means such as pipes or tubes. The mixer 106,either a static mixer or a melt blender or any machine described above,comprises four to six mixing elements which role is to homogenize thepolymer melt in a radial direction. This ensures a high mixing degree ofthe melt which leads to high quality end products.

The mixer 106 does not have any moving parts, i.e. it is a motionless orstatic mixer. It allows low energy consumption, it is free ofmaintenance and does not induce any risk in terms of leakage. It furtherallows predictable homogenisation of the polymer compound, it isrelatively cheap and thus provide a faster return on investment.

The installation 100 further comprises an additive dispensing unit 105which is connected to the cooler 104, to the mixer 106 and/or toconnection means, such as pipes or tubes, which connect together thecooler 104 and the mixer 106. Alternatively, when XLPE for mediumvoltage cables has to be produced, the additive dispensing unit 105 maybe connected directly to the melting machine 101 or the use of feedersdescribed in [0020] (not shown).

The role of the additive dispensing unit 105 is to allow incorporatingperoxide and/or antioxidants and any liquid/molten solid used in watertree retardant formulations or DC formulations or semiconductiveformulations into the polymer compound in liquid form. Peroxide may beadded as a premix of peroxides. Antioxidants may be added as anantioxidant package, which may be prepared in advance. Once the meltcoming out the filtration unit 103 has been cooled down by the cooler104 to peroxide compatible temperatures, around 120° C., both,antioxidant and peroxide, preferably under liquid form, may be injecteddown to the melt, for example via a metering pump. A temperaturecomprised between around 105° C. and around 140° C. is acceptable withregard to the peroxide decomposition, and is thus compatible with thepresent process. In case pre-stabilized polymer is used or freeantioxidant has been added, whether in solid or liquid form, directly inthe melting machine 101 there is no need to add additional antioxidantat that stage. In such a case, a nitrogen blanket is not necessaryeither.

An installation according to a second embodiment of the invention isshown on FIG. 2 .

The installation shares similar components with the installationaccording to the first embodiment but it further comprises a pelletsorting unit 107 arranged before the melting machine 101. Alternativelyor cumulatively, an additional pellet sorting unit (not shown) may bearranged also down the production line, at some stage after the mixer106, preferably just right before a final product, for example XLPEpellets, is conveyed to a packaging unit which packages the finalproduct.

In this embodiment, raw material is thus first acquired by the pelletsorting unit 107 which role is to clean the raw material used by theinstallation. The pellet sorting unit 107 may be set such that anypellet containing a contaminant bigger than 60 pm is discarded beforebeing introduced in the melting machine 101. To reach this goal, thepellet sorting unit 107 includes, in order to detect pellets whichcontain contaminants, one or more CCD cameras, one or more videocameras, X-ray, Ultraviolet or Infra-red detecting means and, in orderto cast away unwanted pellets, a series of air nozzles.

According to a third embodiment of the invention, shown on FIG. 3 , theinstallation shares similar components with the installation accordingto the second embodiment but it further comprises an underwaterpelletizing unit 108, a drying unit 109 and a packaging unit 110.

The underwater pelletizing unit 108 is fed by the molten compound comingfrom the mixer 106 and forced through a die plate. As the compoundemerges from the die plate, pellets are cut in a cutting chamber byrotating blades and are solidified under water which flows across thedie face inside the cutting chamber.

Pellets are then transported to a drying unit 109, for example acentrifugal dryer, where residual water is removed so as to output drypellets.

Further, the packaging unit 110 is adapted to package the product, i.e.the pellets coming out the drying unit 109, for example in van boxes oroctabins. In order to prevent dust to enter in the packaging unit, theoctabins may be assembled outside the packaging unit 110. The octabinsenter the packaging unit 110 through air locks and pellets aredischarged in the octabins. Boxes are further bar coded, identified,given a blend number and conveyed toward an automatic wrapping stationprior to be warehoused.

An installation according to a fourth embodiment of the invention isshown on FIG. 4 .

The installation according to the fourth embodiment shares similarcomponents with the installation according to the first embodiment butthe cooler 104 and the mixer 106 are substituted by a second meltingmachine 111. The second melting machine 111 is equivalent to the firstmelting machine 101 but, instead of working at a temperature levelaround 190° C., it works with a temperature gradient from 190° C., closeto its input, to 130° C., close to its output. The compound coming outthe filtration unit 103 is directly fed into the second melting machine111.

Further, the installation includes an additive dispensing unit 105 whichis connected to the second melting machine 111, preferably close to theend of it.

Since the melting machine is rather long, the temperature of thecompound decreases over the length of the second melting machine 111which allows peroxide to be incorporated.

Preferably, a pellet sorting unit 107 is moreover arranged before thefirst melting machine 101. Alternatively, the installation according tothe fourth embodiment of the invention does not include any pelletsorting unit.

According to a fifth embodiment, shown on FIG. 5 , the installationshares similar components with the installation according to the fourthembodiment but an underwater pelletizing unit 108 is arranged rightafter the filtration unit 103. The underwater pelletizing unit's outputis further connected to a drying unit 109 and a second melting machine111 is arranged right after the drying unit 109.

According to a sixth embodiment, shown on FIG. 6 , an arrangementsimilar to those of FIG. 1, 2 or 3 is set up, wherein the cooler 104 andthe mixer 106 are both replaced by a single unit, such as a PostExtrusion Additive Dispersing Unit (PEAD) 112. Such a unit allows tomanage the temperature, while introducing additives through an additivedispensing unit 105, or any other means, and mixing the obtainedproduct.

In all embodiments of the present invention, the first melting machine101, the melt pump 102, the filtration unit 103, and the pellet soringunit 107, may be arranged in a different order. Alternatively or inaddition, anyone of the pellet sorting unit 107, first melting machine101, melt pump 102 and filtration unit 103 may be omitted. This can bethe case in particular for the melt pump 102. Alternatively or inaddition, two of the pellet sorting unit 107, first melting machine 101,melt pump 102 and filtration unit 103 may be combined. In all theembodiments, the first melting machine 101 may be replaced by anextruder.

The installation and the method of the present invention are applicableto the manufacturing of insulating material comprising semiconductoradditives or not.

All embodiments of the invention allow to produce high qualitycross-linkable polyethylene, in particular cross-linkable polyethylenewhich quality fulfills the legal and technical requirements allowing itto be used for manufacturing insulation parts of medium, high orextra-high voltage power cables without the use of any soaking tower.

EXAMPLE 1

The peroxide/antioxidant dispersion was evaluated for a cross-linkablepolyethylene manufactured according to the method of the presentinvention, and compared to the peroxide dispersion for cross-linkablepolyethylene samples commercially available, as follows:

A Low Density Polyethylene (LDPE) was produced according to thearrangement of FIG. 1 . The antioxidant and the peroxide were premixedbefore injection, and added through the additive dispensing unit 105under liquid form. The temperature of the melted LUPE compound wascomprised between 1 15° C. and 120° C.

50 to 100 of the obtained pellets are collected, press molded into afilm at 1 15° C. in a dedicated frame of 500 pm thick, and quicklycooled down.

The population of analyzed pellets was scanned with a Fourier TransformInfrared Spectrophotometer (FTIR). The additive net absorbance wasdetermined for the two wavelengths 550 cm⁻¹ and 580 cm⁻¹. The filmthickness was determined for each scan by measuring the net absorbanceof the polyethylene band at 2019 cm⁻¹. The additive absorbance wasdetermined using the following equation:Y=net ABS((550 cm⁻¹+580 cm⁻¹)/2)/Net ABS(2019 cm⁻¹)The following parameters are considered:

-   -   n: the sample size or population    -   X: the point estimate, e.g., the sample mean    -   the level of confidence (most commonly 95%)

The chosen confidence, in turns, provides a confidence level coefficientto be fetch according to the Student Table.

-   -   “Stdv”: the sampling variability or the standard error of the        point estimate.

The following expression, provides the 95% confidence interval for thepopulation mean and can be expressed as:X±(Stdv*Student(95%,n−1)(/sqr(n)

The results related to the sample no 1, produced according to thepresent invention, the sample no 2, corresponding to a first commercialcompound produced with a conventional soaking system, and the sample no3, corresponding to a second commercial compound produced with aconventional soaking system, are summarized in table 1 below:

TABLE 1 95% % confidence deviation Sample Standard Stdv % Studentinterval for the against the Sample Size Error against coefficient @population mean. N° n Stdv the mean 95%, n − 1 mean± (X2) 1 100 1.954.59 1.984 0.380 1.8 2 50 10.50 16.72 2.009 3.016 9.6 3 50 4.37 6.752.009 1.255 4.0

The additive distribution is thus much narrow in the compound producedaccording to the present invention, compared to a traditional soakingprocess.

The invention claimed is:
 1. An installation for manufacturing aninsulating part of a medium, high or extra-high voltage power cableusing a cross-linkable polyethylene compound, said installation defininga production path starting at an entry point where raw material isacquired, said installation comprising a first melting machine formelting a polyethylene compound to output a molten polyethylenecompound, a melt pump and a filtration unit for discharging a constantvolume of the molten polyethylene compound, wherein said installationcomprises, in a direction starting at the entry point and going alongsaid production path, a cooler arranged after the filtration unit fordecreasing the temperature of the molten polyethylene compound to alevel which allows a peroxide to be incorporated into the moltenpolyethylene compound without initiating a peroxide scission reaction, amixer arranged after the cooler and an additive dispensing unit forincorporating peroxide in liquid form into the molten polyethylenecompound, wherein the additive dispensing unit is connected with atleast one of the cooler and a connector that connects the cooler and themixer.
 2. The installation according to claim 1, wherein saidinstallation comprises, in a direction starting at the entry point andgoing along said production path, a cooler arranged after the filtrationunit, a mixer arranged after the cooler and an additive dispensing unitconnected with at least one of the cooler and a connector that connectsthe cooler and the mixer, and in that said installation comprises atleast one pellet sorting unit arranged at least one of before and afterthe first melting machine.
 3. The installation according to claim 1,further comprising a debagging unit arranged before the first meltingmachine.
 4. The installation according to claim 1, wherein saidinstallation comprises, in a direction starting at the entry point andgoing along said production path, a cooler arranged after the filtrationunit, a mixer arranged after the cooler and an additive dispensing unitconnected with at least one of the cooler and a connector that connectsthe cooler and the mixer, and in that said installation comprises aunderwater pelletizing unit arranged right after the mixer.
 5. Theinstallation according to claim 1, further comprising a drying unitarranged right after the mixer.
 6. The installation according to claim1, further comprising a debagging unit arranged before the first meltingmachine.
 7. The installation according to claim 1, further comprising adebagging unit arranged before the first melting machine.